Polymer lithium ion batteries represent the state-of-the-art in rechargeable battery technology. A rechargeable lithium battery cell contains an electrolyte through which lithium atoms from a source electrode move between electrodes during charge/discharge cycles. Such batteries are often packaged in a rechargeable lithium-polymer battery pack. This pack utilizes ion conductors having a predetermined formula that is known in the art to provide high energy density, high performance reliability and a prolonged shelf life in a wide range of applications. In the course of the discussion herein, the terms "battery" and "cell" will be used interchangeably.
Plastic lithium ion cells usually consist of a conductive polymer membrane in a lithium salt matrix sandwiched between an anode and a cathode. This type of battery usually contains carbon as an intercalation anode and a metal oxide such as LiCoO.sub.2 or LiMn.sub.2 O.sub.4 as a cathode. A microporous membrane of a conductive polymer such as polyethylene acts as a separator, while solutions of lithium salts (such a LiPF.sub.6) in organic solvents (such as ethylene carbonate and dimethyl carbonate) are used as electrolytes.
Significant weight savings are realized with lithium ion cells which meet the need for reduced battery size and shape in diminutive portable electronic devices. Furthermore, these cells present no electrical hazard when fully charged. Such cells are less sensitive to damage from dropping and the shocks and vibrations of normal use. The cells further demonstrate superior results when subjected to high-rate cycling, mechanical shock, thermal shock, vibration, over voltage, under voltage, short circuits, automobile battery charging, nail penetration and high pressure tests. In addition, there is no liquid to leak and no metallic lithium present, enabling the exercise of safe battery handling procedures. The cell is not an environmental hazard when discarded, and the costs associated with disposal of spent or damaged cells are minimized. Shipping costs are likewise reduced since no special transportation permits or export documents are required.
In the midst of these apparent advantages, the conventional process for manufacturing remains complex and expensive, comprising a multiplicity of independently executed steps. As illustrated in FIG. 1, an electrochemical cell is constructed by individually cutting a plurality of anodes, cathodes and separators (not shown) to the required shapes and sizes. Individual anodes are sandwiched between two separators by heat lamination. The anode-separator assembly is subsequently sandwiched between a pair of cathodes by heat lamination to prepare a plurality of bi-cells 12. Bi-cells 12 are subsequently loaded onto and unloaded from extraction trays to remove excess solvent therefrom. Finally, the desired number of bi-cells 12 are consecutively stacked upon one another to produce make a complete cell 10, as further shown in FIG. 1A.
As it is evident from the above steps, electrochemical cell manufacturing involves many labor intensive handling operations, each of which incurs substantial investments of time and capital. The conventional manufacturing process described above represents a labor-intensive "pick and place" operation that retards the manufacturing process and thereby accelerates the costs associated therewith. Individual handling of bi-cells is not only tedious; it also fosters improper alignment of the components of the bi-cell assembly, leading to improper cell performance and limited yield of the number of cells produced per production run.
Attempts have been made to overcome the problems inherent in the above described mode of manufacture. U.S. Pat. No. 5,456,000 to Gozdz et al., for example, discloses a method of producing lithium-ion rechargeable battery cells. The method includes the steps of arranging in sequence a positive electrode element, a separator element and a negative electrode element. Each of the electrode and separator elements is fabricated from a flexible, polymeric matrix composition substantially devoid of electrolyte salt. Each element is bonded to contiguous elements at a respective interface to form a unitary flexible laminate structure.
Similarly, in related U.S. Pat. No. 5,470,357 to Schmutz et al., an alternative method of making a rechargeable battery structure is disclosed. Such a method includes the steps of arranging contiguously a positive current collector element, a positive electrode element, a separator element, a negative electrode element and a negative current collector element. Each of the electrode and separator elements comprises a flexible, polymeric matrix composition in the form of a self-supporting film. Each of the collector elements comprises a flexible electrically conductive foil which has been surface treated with a compatible polymeric material. Each element is bonded to contiguous elements at the interfaces thereof by applying sufficient heat and pressure to form a unitary flexible laminate structure.
The above-described methods, while increasing the yield of electrochemical cells produced in a manufacturing run, compromise the integrity of such cells so as to render them undesirable for prolonged use in expensive electronic components. The resultant cells consistently suffer from instances of cracking attributable to the folding of elongate film configurations, thereby leading to unpredictable cell imperfections and failures that affect cell performance and reliability.
It is therefore desirable to provide an improved method of manufacturing electrochemical cells which not only amplifies the number of units produced but also optimizes the arrangement of components in each unit while ensuring cell performance and integrity.